If you've ever looked into using a cashback site you will probably have seen feedback from people who didn't get the cash they were promised. Until recently, I thought they had probably done something silly like forgetting to log in first, or hadn't actually paid for the goods. But after my son's experience I started to look into the whole cashback scene and soon discovered it is a consumer rights minefield.

Cashback sites are paid a commission by retailers whenever customers go via them to make a purchase. Customers who sign up are offered a cut of this in the form of cashback if they fulfil two simple requirements:

1. Log in to your cashback account before clicking the appropriate button to take you to the manufacturer's website.

2. Complete your purchase online.

Top Cashback, for example, says that after it has: "traced a paying, applicable transaction to your account, we will credit your account with a 'pending payment'. Once the retailer confirms the transaction, the payment will be marked as 'confirmed'".

When the sum of money amounts to well in excess of £100, it is clearly going to be a factor when customers decide which product they can afford to buy. In my son's case, the monthly payments for a two-year mobile phone contract offering £120 cashback came down from £30 to £25 a month – it was definitely a deciding factor for him.

Yet some websites suggest you should view cashback as a "bonus", presumably to be paid by the company on a whim. That cannot be right. Like a price tag or sale notice in a shop, surely cashback is a promise to pay that should be honoured?

The terms and conditions on most cashback sites state clearly that they are acting as advertising agents for the sellers, and they have a close relationship with these "merchants" as the two work hand-in-glove when it comes to offering inducements to buy their goods and services.

But the sites also say they "cannot be held responsible if the merchant fails to report your sale to us. Merchants have their own rules upon which they determine transactions to be genuine."

And this is where the whole thing goes wrong, because the merchants seem to have an endless stream of reasons why the transaction is not genuine or hasn't tracked.

If an error occurs during the purchasing process you'd think it would just be a matter of submitting a "missing claim" and waiting for things to be sorted out. But in far too many cases this is not an option.

Two years ago Ofcom estimate that cashback mis-selling across the communications industry alone was costing consumers £8m a year, so it is clear problems are not unusual.

My son went through the whole process perfectly normally, and even received an email confirming his contract and telling him the time slot for delivery of his phone. When it didn't arrive he rang the mobile phone company, which said the order had been cancelled. After being assured he would still get his cashback, he placed a new order – but the link back to the cashback site was broken and so no money materialised.

Other people report losing out on cashback because sites have told them they have already paid the commission, but to some unknown third party; claimed that no trackable sale took place at all; or asked for an order number that they then do not recognise.

Yet if you complain to the merchant they will be within their rights to say they never offered you a penny off your purchase, so take no responsibility for rejecting your cashback claim.

What we need is a change in the law so that it is quite clear who is responsible for paying cashback, it promise to pay is legally binding and how customers can get their money back if they have genuinely followed the rules.